In the course of his lifetime, from 1809 to 1882, Charles Darwin wrote several very different books. Some of them, like his monographs on barnacles, were in the ‘worthy but dull’ category. But one of them – The Origin of Species – changed our view of the world. I have read it from cover to cover twice, my two readings being separated in time by about a decade. During this decade – from the early 1970s to the early 1980s – my scientific interests had undergone a major change, from the interface between evolution and ecology to the interface between evolution and development. Because of this change, the two readings were more like reading two different books. The things I noticed second time round had been invisible on my first run through, while the things I had noticed first time round had receded from view by the time I felt compelled to read this extraordinary book again, and were barely noticed on that later occasion.

This ‘two readings becomes two different books’ syndrome is a manifestation of an ancient truth that has been memorably put, though in very different ways, by French microbiologist Louis Pasteur and American folk singers Paul Simon and Art Garfunkel. Pasteur once famously commented that ‘chance favours only the prepared mind’. That is, although luck is a rather random thing, a piece of potentially important information that comes along fortuitously is likely to be overlooked by everyone except those who are in some sense, because of their general interest or their previous studies, predisposed to recognizing its importance.